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NUCLEOTIDE METABOLISM
SITI ANNISA DEVI TRUSDA
Nucleotides are essential for all cells
DNA/RNA synthesisprotein
synthesiscells proliferate
 Carriers of activated intermediates in the
synthesis of carbohydrate, lipids and protein
 Structural component of several essential
coenzymes (coA,FAD,NAD+,NADP+)
 cAMP,cGMP2nd messenger in signal
transduction pathway
 Important regulatory compound for many
of the pathways of intermediary metabolism,
inhibiting/activating key enzimes

Nucleotide structure

Consist of:
◦ Nitrogenous base : purine & pyrimidine
◦ Pentose monosaccharide
◦ 1/2/3 phosphate groups
DNA and RNA contain the same purine bases: A &
G
Pirimidine RNA : U & C
DNA : T & C
T& U differ by only one methyl group
Nucleosides
Pentose sugar + Nitrogen Base = Nucleosides
 So, nucleotides = Nucleosides + Phosphate
 If the sugar is ribose : ribonucleosides
 If deoxyribose: deoxyribonucleosides
 Ribonucleosides of A,G,C,U:
Adenosine,Guanosine,Cytidine,Uridine
 What are the deoxyribonucleosides for
A,G,C,T?

Nucleotides
mono,di,tri esters of nucleosides
 1st phosphate group is attached by an ester
linkage to the 5’OH of the pentosenucleoside
5’phosphate/5’-nucleoside
 Type of pentose is added as prefix for
nucleotide, can be ribose/deoxyribose e.g: 5’ribonucleotide/5’-deoxyribonucleotide

1 phosphate group + 5’-carbon of the
pentosenucleoside monophosphate(NMP) e.g
AMP, CMP
 2 or 3 phosphate group added to the
nucleosidenucleoside di/triphosphate e.g
ADP/ATP
 The latter connected to the nucleotide by a highenergy bond
 Phosphate groups(-) charge
DNA/RNA=nucleic acids


So, what is :
◦ Nucleoside?
◦ Nucleotide?
◦ Nucleic acid?
SYNTHESIS OF PURINE NUCLEOTIDES
Source of purine ring: aspartic acid, glycine,
glutamine, CO2,N10-formylTHF
 Synthesis of 5-phosphoribosyl-1pyrophosphate (PRPP)
an activated pentose for synthesis of
purine/pirimidine & salvage of purine bases
catalyzed by PRPP synthetase, from ATP & ribose
5-phosphate
this enzyme is activated by inorganic phosphat (Pi),
inhibited by purine nucleotides
the sugar of PRPP is ribose ribonucleotides as
end product of purine synthetis

Purine synthesis is critical to fetal
development, therefore defects in enzymes
will result in a nonviable fetus.
 PRPP synthetase defects are known and have
severe consequences (next slide)
 PRPP synthetase superactivity has been
documented, resulting in increased PRPP,
elevated levels of nucleotides, and increased
excretion of uric acid.

Phosphoribosyl Pyrophosphate
(PRPP) Synthetase Defects
PRPP deficiency results in convulsions, autistic
behavior, anemia, and severe mental
retardation.
 Excessive PRPP activity causes gout
(deposition of uric acid crystals), along with
various neurological symptoms, such as
deafness.

Synthesis of 5’-phosphoribosylamine
Amide group of glutamine replaces the
pyrophosphate group at C1 of PRPP
the enzyme, glutamine:phosphoribosyl
pyrophosphate amidotransferase is inhibited
by the purine 5’-nucleotides AMP,GMP,IMP
(end product)
Committed step
Rate of reaction also controlled by
intracellular [] of glutamine and PRPP

Synthesis of inosine monophosphate,the
“parent” of purine nucleotide
requires 4 ATP
2 steps require N10 –formyltetrahydrofolate

Conversion of IMP to AMP and GMP
2 step energy requiring pathway
synthesis of AMP requires GTP as energy
source
synthesis of GMP requires ATP

Conversion of nucleoside
monophosphates to nucleoside di and
triphosphate
AMP + ATP ↔ 2 ADP
GMP +ATP ↔ GDP + ADP
GDP + ATP ↔ GTP + ADP
CDP + ATP ↔ CTP + ADP

Purine Synthesis
Adenilosuksinat
synthetase
IMP dehidrogenase
XMP aminase
Adenilosuksinat
lyase
DAUR dr IMP
 AMP & GMP
Salvage Pathway of purines
Purines that result from the normal turnover
of cellular nucleic acids/diet can be
reconverted into nucleoside
triphosphatessalvage pathway
 2 enzymes: Adenine phosphoribosyltransferase
(APRT), and hypoxanthine-guanine
phosphoribosyltransferase (HPRT)
 Both needs PRPP as the source of the ribose
5-phosphate

Degradation of Purine Nucleotides
Purine Nucleotides from ingested nucleic
acids or turnover of cellular nucleic acids
is excreted by humans as uric acid.
 Humans excrete about 0.6 g uric acid
every 24 hours.
 Degradation of dietary nucleic acids
occurs in the small intestine by pancreatic
enzymes

Digestion of dietary nucleic acids
In the stomach: low pH denatures DNA&RNA
In small intestine: break down phosphodiester
bond by endonuclease (pancreas) 
oligonucleotide
 By phosphodiesterase(exonuclease non spesific
enzyme)  mononucleotide
 By phosphomonoesterase (nucleotidase)
result: nucleoside and orthophosphate.
 Nucleosida phosphorylase result: base and
ribose-1-phosphate.


The nucleoside then absorbed by intestinal mucosal cells
 If the base or nucleoside is unused, it will be reused in salvage
pathways, the base will be degraded:

uric acid
(purin)
ureidopropionic
(pyrimidine).
Diseases associated with purine
degradation
Gout
Elevated uric acid levels in the
blood
 Uric acid crystals will form in
the extremities with a
surrounding area of
inflammation. This is called a
tophus and is often described
as an arthritic “great toe”.
 Can be caused by a defect in
an enzyme of purine
metabolism or by reduced
secretion of uric acid into the
urinary tract.

tophus
Adenosine Deaminase (ADA) and Purine
Nucleoside Phosphorylase (PNP) Deficiency.
accumulation of adenosine wich is converted to
its ribonucleotide or deoxyribonucleotide form by
cellular kinases
 As dATP level rise, ribonucleotide reductase is
inhibited↓ production of all deoxyribose
containing nucleotidescells cannot make DNA
and divide.
 Most severe form: severe combined
immunodeficiency disease (SCID)lack of T and B
cells

A deficiency of either ADA or PNP causes a
moderate to complete lack of immune
function.
 Affected children cannot survive outside a
sterile environment.
 They may also have moderate neurological
problems, including partial paralysis of the
limbs.
 When a compatible donor can be found,
bone marrow transplant is an effective
treatment.

Lesch-Nyhan Syndrome
Hypoxanthine Guanine Phosphoribosyltransferase
(HGPRT) deficiency
 X-linked genetic condition
 Severe neurologic disease, characterized by selfmutilating behaviors such as lip and finger biting
and/or head banging
 Up to 20 times the uric acid in the urine than in
normal individuals. Uric acid crystals form in the
urine.
 Untreated condition results in death within the first
year due to kidney failure.
 Treated with allopurinol, a competitive inhibitor of
xanthine oxidase.

SYNTHESIS OF
DEOXYRIBONUCLEOTIDES
Deoxyribonucleotides required for DNA
synthesis (2’-deoxyribonucleotides)
 Enzyme: ribonucleotide reductase
 Inhibitor : dATP
 Needed a coenzyme : thioredoxin
 Thioredoxin is regenerated by thioredoxin
reductase
 Regulation of ribonucleotide reduction is
controlled by allosteric feedback mechanisms.

PYRIMIDINE SYNTHESIS AND
DEGRADATION
Source of pyrimidine ring: glutamine,
CO2, aspartic acid
 Synthesis of carbamoyl phosphate
from glutamine & CO2, enzyme: carbamoyl
phosphate synthetase II (CPS II), inhibited by
UTP
activated by ATP and PRPP

Synthesis of orotic acid
formation of
carbamoylaspartatedihydroorotateorotic
acid (mind the enzymes!!)
 Formation of a pyrimidine nucleotide :
orotidine 5’-monophosphate (OMP)the
parent of pyrimidine mononucleotide
OMPUridine monophosphate (UMP)
 Synthesis of uridine triphosphate and
cytidine triphosphate
CTP is produced by amination of UTP
 Synthesis of thymidine monophosphate
from dUMP

Orotat
fosforibosiltransferase
CTP synthetase
Orotidilate
dekarboksilase
UMP kinase
Nukleosida diphosphat
kinase
Pyrimidine Synthesis
Production of Uridine
5’-monophosphate
(UMP) from orotate is
catalyzed by the
enzyme UMP synthase
Orotic Aciduria
Deficiency in UMP synthetase activity
 Due to the demand for nucleotides in the process
of red blood cell synthesis, patients develop the
condition of megaloblastic anemia, a deficiency of
red blood cells.
 Pyrimidine synthesis is decreased and excess orotic
acid is excreted in the urine (hence the name orotic
aciduria)

Degradation of pyrimidine
nucleotides

Unlike the purine rings, which are not
cleaved in human cells, the pyrimidine ring
can be opened and degraded to highly
soluble structures, such as β-alanine, and
β-aminoisobutyrate, which can serve as
precursors of acetyl coA and succinyl coA
SALVAGE OF PYRIMIDINES
Pyrimidine salvage defects have not been clinically documented